1. Field of the Invention
The present invention relates to an organic EL (electroluminescence) display apparatus and a driving method therefor.
2. Description of the Related Art
An organic electroluminescence device utilizing electroluminescence (hereinafter abbreviated as EL) of an organic material has a first electrode, a second electrode, and an organic compound layer sandwiched between the electrodes. The organic compound layer includes a light emission layer and a carrier transport layer which are composed of organic molecules. The organic EL device is driven by a current passing between the electrodes. The luminance of the organic EL device is almost exactly proportional to the current (driving current). An organic EL display apparatus in which organic EL devices are arranged in a matrix form has excellent color reproducibility and excellent responsiveness to an input signal, and is therefore ideal particularly for display of moving color images. Furthermore, the organic EL display apparatus can emit light of high luminance and has a wide viewing angle, and therefore can be used in various environments. As a material used for the organic compound layer, there are low-molecular-weight materials ideal for vacuum deposition, oligomer and polymer materials ideal for spin coating and ink-jet coating. Currently, low-molecular-weight materials are in widespread use. However, oligomer and polymer materials, which are ideal for display on a large screen, will probably be used increasingly in the future.
Examples of a pixel driving method are the passive matrix method and the active matrix method. In the passive matrix method, a current is directly passed between first electrodes formed in a striped pattern and second electrodes formed in a striped pattern, the striped patterns being orthogonal to each other, so as to cause organic EL devices sandwiched between the first and second electrodes to emit light. In the active matrix method, pixel circuits each composed of thin-film transistors (hereinafter abbreviated as TFTs), a capacitor, etc. and each used to drive an organic EL device, are arranged in a matrix form. Image signals are individually transmitted to pixels, and are then maintained in corresponding pixel circuits. Organic EL devices emit light in accordance with the maintained pixel signals, thereby displaying an image. In the case of the active matrix method, image signals to be transmitted to individual pixels are rarely mixed. Accordingly, this method is ideal particularly for a display apparatus with a large screen, high definition, and a large number of pixels.
The active matrix driving method is roughly classified into the voltage programming method and the current programming method. In the voltage programming method, a potential, which serves as an image signal, is directly applied to the gate of a driving TFT and is then maintained. A current passing through the driving TFT is controlled by the potential of the gate thereof. However, the relationship between the current and the potential of the gate varies according to the TFT, and sometimes changes with operating time. Accordingly, in the case of the voltage programming method, luminance is prone to vary from pixel to pixel, and image burn-in is prone to occur. On the other hand, in the case of the current programming method, a current, which serves as an image signal, is passed through a driving TFT included in each pixel just before an image is displayed, and the gate potential of the driving TFT at that time is maintained. Accordingly, variation of driving TFT characteristics and a change in driving TFT characteristics with time have little effect on display of an image compared with the voltage programming method.
FIG. 3 shows an example of a driving circuit compliant with the current programming method disclosed in U.S. Pat. No. 6,373,454. This driving circuit includes pixel circuits 100, first constant voltage sources 101, second constant voltage sources 102, signal lines 103, and signal current sources 104 connected to the signal lines 103.
Each of the pixel circuits 100 includes an organic EL device 106 one of whose electrodes is connected to one of the first constant voltage sources 101, a driving TFT 107 whose drain is connected to the other electrode of the organic EL device 106, a voltage maintaining unit 108 for maintaining a gate-to-source voltage of the driving TFT 107, a first switch 109 disposed between the gate and drain of the driving TFT 107, a second switch 110 disposed between the source of the driving TFT 107 and one of the signal lines 103, and a third switch 111 disposed between the source of the driving TFT 107 and one of the second constant voltage sources 102.
In a programming period, that is, a signal write period, the first switch 109 and the second switch 110 are closed and the third switch 111 is opened so as to provide a signal current for the source of the driving TFT 107 in accordance with an image signal transmitted from one of the signal current sources 104. The source-to-gate voltage at that time is maintained in the voltage (capacitance in FIG. 3) maintaining unit 108 disposed between the source and the gate.
In an image display period, the first switch 109 and the second switch 110 are opened, and the third switch 111 is closed. Consequently, a current passes through the driving TFT 107 in accordance with the source-to-gate voltage determined and maintained in the signal write period, whereby the organic EL device 106 emits light.
The driving TFT 107 shown in FIG. 3 is a p-channel TFT. The drain terminal of the driving TFT 107 is connected to the anode of the organic EL device 106, so that a current passes from the drain to the organic EL device 106. If the driving TFT 107 is an n-channel TFT, the positions of the source and drain may be interchanged. The drain terminal may be connected to the cathode of the organic EL device 106, so that a current passes from the organic EL device 106 to the drain. This example is disclosed in U.S. Pat. No. 6,229,506.
The pixel circuits 100 are formed on a glass substrate using amorphous silicon or polysilicon. However, a metal oxide such as InGaZnO disclosed in WO 05/088726 may be used.
An organic EL display apparatus is often used in mobile apparatuses such as mobile telephones or digital cameras. Accordingly, power consumption is required to be reduced. In order to reduce power consumption during the image display period, it is advantageous that a power supply voltage (a voltage between the first and second constant voltage sources in FIG. 3) is reduced.
In the case of a known pixel circuit compliant with the current programming method, a driving TFT is diode-connected at the time of current programming. Subsequently, a signal current is externally provided for the driving TFT, whereby a gate-to-source voltage of the driving TFT is determined and maintained. In the image display period, a current that is the same as the signal current is passed through an organic EL device in accordance with the maintained gate-to-source voltage.
When the organic EL device emits light at the maximum luminance, the gate-to-source voltage of the driving TFT and the current passing through the organic EL device also become maxima. At least the sum of the voltage across the organic EL device when it emits light at the maximum luminance and the gate-to-source voltage of the driving TFT at that time is required as a power supply voltage. A voltage lower than the sum cannot be used.
In order to further reduce power consumption of the organic EL display apparatus, a new driving circuit is required instead of the known driving circuit compliant with the current programming method.
Like the above-described power supply voltage, a voltage of an output terminal of a signal current source also becomes the maximum value when the organic EL device emits light at the maximum luminance. Accordingly, at least the sum of the voltage across the organic EL device when it emits light at the maximum luminance and the gate-to-source voltage of the driving TFT at that time is required as a power supply voltage used to drive the signal current source. From the viewpoint of power saving, the power supply voltage for the signal current source is preferably reduced.